RadioGraphics
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
QUICK SEARCH:   [advanced]


     

DOI: 10.1148/rg.281075024
This Article
Right arrow Abstract Freely available
Right arrow Full Text
Right arrow Full Text (PDF)
Right arrow Submit a response
Right arrow Alert me when this article is cited
Right arrow Alert me when eLetters are posted
Right arrow Alert me if a correction is posted
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Bauhs, J. A.
Right arrow Articles by McCollough, C. H.
Right arrow Search for Related Content
PubMed
Right arrow Articles by Bauhs, J. A.
Right arrow Articles by McCollough, C. H.
Related Collections
Right arrow Physics and Basic Science
Right arrow Computed Tomography

CT Dosimetry: Comparison of Measurement Techniques and Devices1

John A. Bauhs, PhD, Thomas J. Vrieze, RT(R), Andrew N. Primak, PhD, Michael R. Bruesewitz, RT(R), and Cynthia H. McCollough, PhD

1 From the Department of Radiology, Mayo Clinic, 200 First St SW, Rochester, MN 55905. Presented as an education exhibit at the 2006 RSNA Annual Meeting. Received February 16, 2007; revision requested April 11 and received June 7; accepted June 11. J.A.B. receives research support from RTI Electronics; A.N.P. receives research support from Siemens; C.H.M. receives research support from Siemens and RTI Electronics; all other authors have no financial relationships to disclose.

Figure 1
View larger version (13K):
[in this window]
[in a new window]
[Download PPT slide]
  		Figure 1.  The radiation dose profile along a line perpendicular to the plane of a single axial CT scan shows a peak where the primary beam slices through the CTDI phantom. The tails of the dose profile are caused by scattered radiation. The integral of the area under the curve is normalized to the nominal beam width NT to determine the CTDI. A CTDI100 value is obtained if integration limits of ±50 mm are used.

 

Figure 2
View larger version (28K):
[in this window]
[in a new window]
[Download PPT slide]
  		Figure 2.  The radiation dose profiles from nine adjacent axial CT scans along a line perpendicular to the axial scans, when summed, produce the MSAD profile. The value of MSAD is averaged over one scan interval in the central portion of the profile. In contrast to the CTDI, which can be used to estimate MSAD, the MSAD is measured with many TLDs and multiple CT scans.

 

Figure 3
View larger version (78K):
[in this window]
[in a new window]
[Download PPT slide]
  		Figure 3.  Equipment typically used to measure CTDI100 includes an integrating electrometer (black arrow), a 100-mm-long CTDI ionization chamber (white arrow), and a CTDI phantom made of polymethylmethacrylate (arrowhead). The phantom is placed with its long axis perpendicular to the plane of the axial CT scan, and the ionization chamber is placed in one of the holes through the phantom. The CTDI100 is obtained by integrating the dose from a single axial scan and dividing by the nominal beam width.

 

Figure 4A
View larger version (113K):
[in this window]
[in a new window]
[Download PPT slide]
  		Figure 4a.  (a) An OSL dosimeter (arrow) is inserted into a head CTDI phantom to measure the CTDI100. The probe is exposed to a single axial scan, then returned to the manufacturer for reading with a precision scanning laser. (b) The resultant radiation dose profile information is returned to the user in both print and digital formats.

 

Figure 4B
View larger version (27K):
[in this window]
[in a new window]
[Download PPT slide]
  		Figure 4b.  (a) An OSL dosimeter (arrow) is inserted into a head CTDI phantom to measure the CTDI100. The probe is exposed to a single axial scan, then returned to the manufacturer for reading with a precision scanning laser. (b) The resultant radiation dose profile information is returned to the user in both print and digital formats.

 

Figure 5A
View larger version (113K):
[in this window]
[in a new window]
[Download PPT slide]
  		Figure 5a.  (a) A solid-state real-time dosimeter (arrow) is inserted into a head CTDI phantom to measure the CTDI100. The probe and phantom are passed through the x-ray beam by using the motion of the patient table in a spiral acquisition mode; a real-time electrometer records the dose delivered to a submillimeter-thick dosimeter in the center of the probe. (b) The CT dose data are collected by a computer and used to generate a dose profile, which is integrated and normalized to the nominal beam width, thus giving the CTDI100. If central axis and peripheral dose data are collected from consecutive scans, the software will automatically calculate CTDIvol.

 

Figure 5B
View larger version (71K):
[in this window]
[in a new window]
[Download PPT slide]
  		Figure 5b.  (a) A solid-state real-time dosimeter (arrow) is inserted into a head CTDI phantom to measure the CTDI100. The probe and phantom are passed through the x-ray beam by using the motion of the patient table in a spiral acquisition mode; a real-time electrometer records the dose delivered to a submillimeter-thick dosimeter in the center of the probe. (b) The CT dose data are collected by a computer and used to generate a dose profile, which is integrated and normalized to the nominal beam width, thus giving the CTDI100. If central axis and peripheral dose data are collected from consecutive scans, the software will automatically calculate CTDIvol.

 

Figure 6A
View larger version (19K):
[in this window]
[in a new window]
[Download PPT slide]
  		Figure 6a.  There is good agreement between dose profiles generated by the OSL dosimeter shown in Figure 4 (solid line) and the solid-state detector shown in Figure 5 (dashed line). The graphs show radiation dose profiles for probes placed in air at the isocenter of the CT scanner (a) and probes placed in the center of a CTDI head phantom (b).

 

Figure 6B
View larger version (20K):
[in this window]
[in a new window]
[Download PPT slide]
  		Figure 6b.  There is good agreement between dose profiles generated by the OSL dosimeter shown in Figure 4 (solid line) and the solid-state detector shown in Figure 5 (dashed line). The graphs show radiation dose profiles for probes placed in air at the isocenter of the CT scanner (a) and probes placed in the center of a CTDI head phantom (b).

 

Figure 7
View larger version (17K):
[in this window]
[in a new window]
[Download PPT slide]
  		Figure 7.  Left: In perfusion or interventional CT (where there is no table movement), the peak skin dose is the relevant dose parameter for deterministic skin effects. The peak skin dose equals the peak dose from one scan times the number of scans. Right: If the peripheral CTDI100 is used as a surrogate for peak dose (as prescribed by IEC standard 60601-2-44), the skin dose will be overestimated by up to a factor of two. Because the definition of CTDI assumes table incrementation, CTDI is not a valid parameter for scans performed without table movement.

 

Figure 8A
View larger version (113K):
[in this window]
[in a new window]
[Download PPT slide]
  		Figure 8a.  The real-time solid-state dosimeter shown in Figure 5 was used to measure both peripheral CTDI100 and point dose for spiral and perfusion CT examinations of the head and thorax. The center detector of the probe (arrow) was positioned over the lens of the eye (a) and the upper thorax (b). For the chest examination, tissue-equivalent bolus material was layered over the probe. Clinical spiral and perfusion CT scans were performed at each location, and both the CTDI100 and point dose were measured.

 

Figure 8B
View larger version (126K):
[in this window]
[in a new window]
[Download PPT slide]
  		Figure 8b.  The real-time solid-state dosimeter shown in Figure 5 was used to measure both peripheral CTDI100 and point dose for spiral and perfusion CT examinations of the head and thorax. The center detector of the probe (arrow) was positioned over the lens of the eye (a) and the upper thorax (b). For the chest examination, tissue-equivalent bolus material was layered over the probe. Clinical spiral and perfusion CT scans were performed at each location, and both the CTDI100 and point dose were measured.

 




HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
RADIOGRAPHICS RADIOLOGY RSNA JOURNALS ONLINE
Copyright © 2008 by the Radiological Society of North America.